Sensors (and actuators) have usually at least two microcontrollers, wherein e.g. in the case of a sensor a process side microcontroller is associated with the sensor element and a transmitter side microcontroller is associated with the measurement transmitter. As a rule, measurement data representing the process variable are transmitted from the process side microcontroller to the transmitter side microcontroller, while mostly parameter data or sensor-specific identifying data are transmitted from the transmitter side microcontroller to the process side microcontroller. The data transmission, respectively communication, between the two microcontrollers is usually asynchronous. The communication occurs e.g. via a UART interface.
Problematic in the case of the asynchronous communication is that the clock frequency of a microcontroller is dependent on temperature fluctuations and voltage oscillations. Consequently in the case of a too strong relative drift of the clock frequencies of the two microcontrollers, an asynchronous communication or a time/frequency measurement can become unusable. It is thus necessary to keep the internally produced clock frequencies of the microcontrollers stable.
Usual in this connection is to keep the clock frequency, respectively the clock signal, of a microcontroller stable via an additional, stable clock frequency. For example, the additional stable clock frequency is provided by a clock crystal, wherein a clock crystal is distinguished by a lower temperature dependence. Disadvantageous in the case of the known solution is the extra costs caused by the additional clock crystal. Furthermore, the clock crystal requires additional space, a feature which is problematic in many fields of technology—for instance in automation technology—due to the continuing drive to miniaturize the sensors.